1. Field of the Invention
This invention relates to weapons for attacking rotorcraft and more particularly to weapons that home in on the acoustic signatures of these craft and then attack their targets by deploying cables that break or foul rotor blades and/or their pitch adjustment mechanisms.
2. Prior Art
Rotating wing aircraft have become effective instruments of modern warfare not only because of their maneuverability, but also because they are shielded from conventional line-of-sight weapons that are aimed and/or guided optically, or by radar or infrared during nap-of-earth flight, and can limit their exposure above that shield to times too short to accommodate effective aiming and firing of these line-of-sight weapons. Such craft are, however, ideal targets for acoustically guided weapons, not only because their distinctive acoustic signatures are recognizable despite nap-of-earth flight, but also because acoustically guided weapons can, despite limits imposed by acoustic guidance systems, fly faster than rotating wing aircraft because these craft are limited to even lower speeds by the loss of lift that occurs when advancing blades approach sonic velocity.
Weapons responsive to acoustic signatures are well known from the art of naval warfare, and Kelsay, U.S. Pat. No. 2,766,713, discloses a Torpedo-Steering Control System in which the outputs of a pair of axi-symmetrically displaced microphones are compared, the position of the torpedo rudder being adjusted to make the acoustic signals at both microphones the same. While Kelsay's arrangement addresses only yaw, three-dimensional steering can obviously be effected by adding a second such system, the microphones and control surface or surfaces of which are orthogonal with respect to the first. Alternatives to Kelsay's two-microphone arrangement are also known, and Turner, U.S. Pat. No. 2,407,330, discloses a system for determining the bearing of an acoustic source by rotating a single directional microphone, and Ziehm et al, U.S. Pat. No. 3,496,527, disclose a single transducer assembly that fixes direction but does not require rotation.
Prior art acoustically guided weapons are typically torpedoes, the targets of which are ships, submarines, etc., the hulls of which are large contiguous acoustic radiators that are effectively attacked by impact-detonated weapons that home in on the acoustic signatures of their targets. Rotating wing aircraft are, on the other hand, mostly "open space", and the rotating blades, which describe a large circle and are the most vulnerable elements of these craft, occupy only a small portion of that circle at any particular instant of time. Further, the characteristic acoustic signatures of such craft are partially the result of the slap that results when the retreating blade loses lift and partly the result of blade-fuselage interaction, and neither of these dominant effects emanates directly from a well defined acoustically targetable point. Thus acoustically guided anti-rotorcraft weapons must either close optically or by infrared or radar, each of which has countermeasures, be detonated by proximity and carry a large enough warhead to be effective at a distance, or as preferred here, by deploying cables that break and/or foul rotor blades and/or their pitch-adjusting mechanisms.
Cables were suspended from balloons for protection against aircraft during World War II, but such arrangements would not be effective now because they are radar-detectable, and once detected are easily avoided by craft as maneuverable as helicopters.
Cable assemblies have also been proposed as "kinetic kill" defenses against ICBMs, the cable assemblies being deployed as rotating arrays with their planes of rotation normal to the paths of these missiles. This rotation not only maintains the collision cross-section of the array, but also facilitates the destruction of missiles that, because of their shape, might otherwise pass between the elements of that array. Cables that are deployed to destroy rotating wing aircraft do not have to be rotated, however, because these craft have adequate collision cross-sections that result from rotation of their most vulnerable elements. Because the plane of that rotation is substantially horizontal, the cables of the present invention are deployed in substantially vertical configurations that are more effective against their targets.
These substantially vertical configurations are not established instantaneously, however, and cannot be deployed well in advance of arrival of their targets for reasons mentioned earlier, so that circuitry, the prototype for which is that disclosed by Owen et al, U.S. Pat. No. 4,409,899, is used to initiate cable deployment early enough to attack targets most effectively.
In one embodiment of this invention, cables are deployed from their carrier missiles by rockets, and Pinson, U.S. Pat. No. 4,625,646, discloses an arrangement in which submissiles are deployed from carrier missiles. Pinson's submissiles, however, not only deploy no cables, but are launched with a rearward orientation with respect to the carrier missile, so that, even if cables were to be deployed by Pinson's submissiles, the cables would be ineffective because they would be parallel to rather than normal to the planes of rotation of rotorcraft rotors.
The missiles of this invention travel at subsonic velocity, at least during the acoustically guided portion of their flight, and for this reason they are patterned after anti-tank rather than anti-aircraft missiles, and the anti-tank missiles of particular relevance to the present invention are the HUGHES BGM-71 TOW that has folding winglets and tail fins and a two stage propulsion system, and the MCDONNEL DOUGLAS M47 DRAGON that has sequentially fired sustainer rockets. These prior art anti-tank missiles are described in The Encyclopedia of World Military Weapons, Bishop, C. & Drury, I. Crescent Books, New York, 1988, pp 203-4.